This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The projects being pursued try to establish the structural bases for the functional mechanisms by which the ribosome and associated factors achieve their roles in protein synthesis by high resolution X-ray crystallography. To illuminate the mechanisms of each step in the process of protein synthesis, we wish to obtain crystals of the ribosome trapped in as many of the steps in the protein synthesis cycle as possible and to establish their structures at the highest resolution possible. The complexes whose structures will be pursued include those of the 1) 70S ribosome with bound m-RNA, tRNA and elongation factors, 2) complexes between the 70S ribosomal and a P-site tRNA containing a translation arresting polypeptide, and 3) complexes between the 70S ribosome the signal recognition particle. To explore the differences between the bacterial ribosome and its much large counterpart from eukaryotes, particularly in the initiation step of protein synthesis, the structures of the 40S subunit, the 60S subunit and/or the SOS ribosome, including an initiation complex formed with IRIS RNA, and using ribosomes isolated from either yeast or rabbit reticulocyte will be pursued.